This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-390472 filed Dec. 22, 2000.
The present invention relates to a driving apparatus for a piezo-actuator, particularly to a driving apparatus for a piezo-actuator having an abnormality detecting function.
A piezo-actuator produces displacement in accordance with an applied voltage by expansion and contraction of a piezoelectric body. It provides high response and excellent control performance. Therefore, it is proposed to utilize the piezo-actuator, for instance, a piezo-injector, in a vehicular driving apparatus.
Meanwhile, in recent years, in order to deal with new vehicular regulations, it is necessary to establish a failure diagnosis technology of vehicle-mounted apparatus. In a piezo-actuator which generates a high voltage, it is necessary to stop driving the piezo-actuator swiftly upon occurrence of abnormality.
As a countermeasure against high voltage of a piezo-actuator, an abnormality detecting circuit is provided in a driving apparatus for a piezo-actuator. For example, JP-A-3-128679 discloses a driving apparatus which applies a high voltage for a predetermined time period in contracting a piezo-actuator and discharges accumulated electric charge. This apparatus detects whether the discharging has been completed. When the discharging has not been completed, supply of high voltage is prohibited thereafter and electric charge of the actuator is discharged. The apparatus thus ensures safety by discharging electric charge when some abnormality occurs in driving the actuator.
Further, it is proposed to detect a voltage level (or current) of an output terminal controlled by ON/OFF signals and determine abnormality based thereon. That is, when a terminal of a suck-in output is turned on, the voltage level is L level. When turned off, it is H level. At a flow-out terminal, the condition is reversed. Therefore, by comparing the ON/OFF signal and detected voltage level, abnormality can be determined. When abnormality is detected based on current, abnormality is determined by whether the current reaches a prescribed current at ON/OFF-time. Further, when abnormality is detected, high voltage generating operation is prohibited and the abnormality is displayed.
However, there are various abnormal situations such as disconnection or short-circuit of an output terminal, disconnection, partial disconnection or short-circuit, current leakage of a piezo-actuator and the like. Particularly, in the case of a switching method of carrying out charge and discharge in stages, it is also likely that charge and discharge are carried out by interruption caused by contact failure of wires. Thus, a number of circuits are needed to deal with all possible abnormalities. A piezo-injector for multi-cylinder engine is provided with output terminals of both of flow-out output and suck-in output, and circuits for a number of cylinders are needed. Therefore, the circuit construction becomes complicated and cost is increased.
It is an object thereof to provide a driving apparatus for a piezo-actuator capable of easily and accurately detecting abnormality without complicating a circuit construction or increasing cost.
According to the present invention, a driving apparatus for a piezo-actuator detects abnormality by detecting either or both of the numbers of times of switching in charging and in discharging and charging/discharging time periods and comparing the detection results with those in normal time.
In driving the piezo-actuator, a charging or the discharging switching element is controlled to be turned on/off such that currents flowing in charging and discharging are constant and the numbers of times of switching in charging and in discharging are equal to each other and become a substantially constant prescribed number of times. However, when some abnormality occurs in charging/discharging paths, this relationship does not hold. Therefore, the abnormality can be determined when the numbers of times of switching in charging and in discharging are considerably smaller or larger than the prescribed number of times, or when a difference between the numbers of times of switching in charging and in discharging is large. Further, charging/discharging time periods also become a substantially predetermined time period. Therefore, when the predetermined time period is considerably exceeded, abnormality can be determined. The abnormality can be determined comparatively easily based on the numbers of times of switching in charging and in discharging, or the charging/discharging time periods. Thus, a complicated detecting circuit is not needed and cost is not increased.